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Air consists roughly of $N_2$, $O_2$ and $Ar$.

Why is $Ar$ the relatively common noble gas compound and not for example the lighter ones $He$ or $Ne$?

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Argon is produced when Potassium 40 decays. The majority of Argon in Earth's atmosphere comes from this source. We know this because K-40 decay produces Ar-40, and the majority of Ar in the atmosphere is Ar-40. In the sun the majority is Ar-36.

JR

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I should add that the lighter an element the more probable it is to go towards the stratosphere (buoyancy) and to have attained escape velocity at the high edge of the Maxwell Boltzmann distribution. –  anna v Jan 24 '12 at 10:02
    
This answer seems a bit complete. Air is mostly argon because Argon is produced by Potassium 40 decay, but there is no explanation of why this decay process occurs more commonly or in greater magnitude than decays that would result in helium or oxygen or other constituent gases of earth's atmosphere. –  Lisa Oct 17 '12 at 0:38
    
I think the only other gas produced in significant quantities by radioactivity is helium. I'm not sure how the production rates of argon and helium compare, but helium is rapidly lost from the atmosphere because it's so light. –  John Rennie Oct 17 '12 at 6:25

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